Though nearly the size of Earth, Acaeria contains only 28% of its mass. It has a trace atmosphere of neon and molecular nitrogen, but the predominant carbon dioxide has long since frozen and fallen to the surface as frost. While Acaeria has a core of heavy metals, the bulk of the planet's volume consists of water ice. Several unique forms of long-chain carbon molecules have been recovered on the surface, pushed up from beneath the ice by cryovolcanic processes. Acaeria has a large rocky moon, compositionally similar to Luna.